Internal combustion engines produce undesirable exhaust emissions, including CO, CO2 and NOx. Vehicle engines are required to comply with legislative limits which prescribe the level of permitted emissions, typically over a standard driving cycle. The level of such emissions is being continually reduced.
A typical gasoline engine uses catalytically coated substrates in each exhaust system to minimize tailpipe emissions. Effective operation of these exhaust catalysts typically requires that the exhaust gas they are presented with is maintained at near to stoichiometric air:fuel ratios. In support of this the engines are provided with closed loop feedback control of fuelling, whereby an oxygen sensor in the exhaust tract determines whether the exhaust gases have an oxygen content indicative of non-stoichiometric combustion. The sensor output is used to continually adjust fuelling of the engine to compensate for a lean or rich mixture—thus gas flow through the catalyst is generally maintained at or close to stoichiometric, and undesirable emissions can be minimized.
Considerable advances have been made in closed loop feedback control of fuelling, but this approach can only correct fuelling after a departure from the target air:fuel ratio has been identified. Accordingly it is possible that if a large disturbance is experienced in the exhaust gas air:fuel ratio the storage capacity of the catalysts may be exceeded and some undesirable emissions may pass to atmosphere even if closed loop feedback control is fast and accurate.
Vehicle engines may have selectable features to permit operation in alternative modes. For example a dual camshaft arrangement can provide for low valve lift and higher valve lift to give a wider range of cam timing relationships over the engine load/speed map. A diagnostic is required to confirm operation of the correct mode, because otherwise the engine will have inappropriate valve timing and for example may be inappropriately fuelled; as a consequence undesirable emissions may not be adequately controlled.
A diagnostic may for example rely upon analysis of exhaust gas during a momentary forced change of camshaft condition, whereby for example high lift mode is selected, thus allowing a different quantity of air to enter the respective cylinders. Combustion in these cylinders will be affected, with a consequent effect upon exhaust gas constituents. Thus in this example a change in exhaust composition can indicate correct operation of a high lift camshaft condition, whereas unchanged composition can indicate a malfunction.
A diagnostic of this kind must be periodically repeated in order to provide a regular check on correct operation of a cam profile switching arrangement. Legislation may require that operation of the diagnostic be recorded in a vehicle system, for subsequent review and/or analysis.
Each time the diagnostic is performed, a momentary increase in undesirable emissions may occur since it is the change in exhaust constituents that allows correct cam switching to be confirmed. The diagnostic is preferably performed for the minimum time period to give a reliable reading at the oxygen sensor, but nevertheless an increase in undesirable emissions could still occur.
What is required is a diagnostic which further reduces the possibility of undesirable emissions, yet uses the existing strategy of a momentary forced change of camshaft condition.